Animation with python NLA tracks failed
There are a couple of scripts in one rig = bpy.context.active_object in the other rig = bpy.context.active_object.data.materials[0]  I'm assuming you are on the first as the second (materials one) creates the animation_data.

The objects are taken from the context.selected_objects collection, by  name, then sorted, then iterated and made active.

However if you have an object selected that doesn't have any animation_data, ie has no action with keyframes nor a driver, then the script will fail because obj.animation_data is None and as the error says


  AttributeError: 'NoneType' object has no attribute 'nla_tracks'  


One fix would be to replace

objs = bpy.context.selected_objects


with 

objs = [o for o in bpy.context.selected_object if o.animation_data]


or go straight to names

names = [o.name for o in bpy.context.selected_objects
            if o.animation_data]


to make sure each object, from selected_objects collection, has animation data.